The main types are:

• Sapphire glass;
• Mineral glass;
• Acrylic (Plexiglas), transparent plastic.

Sapphire

Sapphire is extremely hard and scratch-resistant. This makes it a good choice for many watches. The sapphire used for watch glasses is usually artificially synthesized.

K-1 glass

K1 is a processed glass with a higher resistance to scratching and breaking from the mineral glass.

Mineral glass

Probably the most popular material for watch crystals. Generally speaking, mineral glass is usually glass that is chemically treated or by temperature to make it more scratch-resistant. Depending on the processing, It may posses characteristics similar  those of the sapphire.

Acrylic

Acrylic is easily scratched and used only in low-end watches.